1. Field of the Invention
This invention relates to a method for the removal of mercury contained in a gas. More particularly, this invention relates to a method for efficiently removing a mercury vapor which is present in a small amount in a sulfur dioxide-containing gas.
2. Description of the Prior Art
Hitherto, a wide variety of methods have been proposed for the removal of a small amount of mercury vapor present in a gas using a washing solution. Typical conventional procedures include a method using an acidic oxidative solution such as an aqueous solution, for example, an aqueous sulfuric acid solution of manganese peroxide as a washing solution, a method using an aqueous solution containing divalent mercury ion and an anion such as chlorine, bromine or sulfuric acid ion as a washing solution, and a method using an aqueous sulfuric acid solution having a high temperature and a high sulfuric acid concentration as a washing solution.
However, there are various problems in these conventional procedures when they are applied to a sulfur dioxide-containing gas for the removal of a small amount of mercury vapor present in the gas. That is, the use of an oxidative acidic solution of sulfuric acid and manganese peroxide as a washing solution easily results in the reduction of oxidizing agents, i.e., manganese peroxide by sulfur dioxide present in the gas to be washed thereby remarkably decreasing the effect on the removal of the mercury vapor from the gas. The use of an aqueous solution containing divalent mercury ion and an anion such as chlorine, bromine, iodine or sulfuric acid ion, etc. as a washing solution also results in the reduction of a part of the divalent mercury ion present in the solution to a monovalent mercury ion by a sulfur dioxide in the gas, as is described above for the method comprising washing the gas with an oxidative solution, thereby remarkably decreasing the effect on the removal of the mercury vapor from the gas. The use of a sulfuric acid solution having a high temperature and a high sulfuric acid concentration as a washing solution generally does not adversely affect the effect on the removal of a mercury vapor by a sulfur dioxide present in the gas, but tends to cause corrosion, damage, etc. of the apparatus due to the high temperature and high concentration of sulfuric acid thereby making the method uneconomical in practical use.